Proposals to use induced gamma ray spectroscopy for detecting carbon/oxygen ratio in a well borehole dates back several decades. Commercial operations in this field have dated from the mid-1970's. The primary applications today remain the cased hole determination of oil saturation in reservoirs of low or unknown water salinity. Well boreholes are usually "cased" soon after drilling by inserting tubular steel casing, the inside diameter of which is usually filled with oil or water or gas or a combination thereof. Void space between the outside of the casing and the earth formation penetrated by the borehole is filled with a sheath of cement. Recent efforts have expanded the use of the technique to geochemical analysis in open hole. Unfortunately, the intrinsic dynamic range of the carbon/oxygen ratio measurement is small. This requires very high statistical precision for reasonable oil saturation uncertainty. Until recently, the technique has been very slow and often required stationary measurements. This was in part because of the small intrinsic dynamic range of the carbon oxygen ratio, but, also because the detectors used in well logging instruments had significant limitations.
A gamma ray scintillation type detector consists of a scintillation crystal optically coupled to a photomultiplier tube. Intensity of light induced within the crystal by an impinging gamma ray is proportional to the energy of the gamma ray. The optically coupled photomultiplier tube generates an electrical pulse which is proportional to the intensity of the light generated within the scintillation crystal. It follows, therefore, that the electrical pulse generated by the photomultiplier tube is proportional to the energy of the gamma ray impinging upon the scintillation crystal.
Over the past decade or so, several new high density scintillators in gamma ray spectrometer detectors have become available which can provide improved gamma ray detection capability. Newer carbon/oxygen and geochemical logging tools can use these new high density scintillators. One of these materials is bismuth germanate (BGO). The merit of this higher density scintillator relative to less dense conventional sodium iodide scintillator (previously used in induced gamma ray spectrometers) is that it provides substantially better detection efficiency. That is, a larger fraction of the total gamma ray flux is in the full energy or first escape peak. Less gamma ray energy is disposed in the structureless Compton scattered low energy tail, in the more dense bismuth germanate detectors relative to sodium iodide. Additionally, in a well logging system according to concepts of the present invention, a different and unique gain stabilization system is employed which further insures more accurate energy representation of the detected gamma rays by more precisely controlling the system gain.